doc: fix comment about String.fromUTF8 replacing invalid chars (#10240)

Hi, the doc of `String.fromUTF8` previously said invalid characters are
replaced with 'A'. But the parameter `h : validateUTF8 a` guarantees
there are no invalid characters, so that explanation doesn't make sense
to me. This PR deletes that explanation (and fixes some unrelated
typos).

I also have a patch that uses `h` to prove each of the characters is
valid, eliminating the need for a default character
([pr/chore-String-fromUTF8-prove-valid](27f1ff36b2)),
would you be interested in merging that?

<details>
<summary>Notes on invalid characters from unchecked C++</summary>
I don't know if this function may be called from unchecked C++ with
invalid characters. If it may, I'm not sure what would happen with my
patched function... I'm not familiar with Lean's safety model, but it
seems like a bad idea to have a Lean function that takes a proof of a
proposition but is expected to operate in a certain way even if the
proposition is false. I think the safe approach is to have two functions
-- one that takes a proof and is only called from Lean, and another that
doesn't take a proof and replaces invalid chars (for use from C++, not
sure whether it's useful from Lean); I'd prefer to go even further and
report an error instead of silently replacing invalid characters (I'm
not sure if there is any easy way to report errors/panic in Lean code
called from C++).
</details>

src/Init/Data/String/Extra.lean

@@ -104,8 +104,7 @@ where
 
 /--
 Decodes an array of bytes that encode a string as [UTF-8](https://en.wikipedia.org/wiki/UTF-8) into
-the corresponding string. Invalid UTF-8 characters in the byte array result in `(default : Char)`,
-or `'A'`, in the string.
+the corresponding string.
 -/
 @[extern "lean_string_from_utf8_unchecked"]
 def fromUTF8 (a : @& ByteArray) (h : validateUTF8 a) : String :=

src/Init/Tactics.lean

@@ -1025,7 +1025,7 @@ The form
 ```
 fun_induction f
 ```
-(with no arguments to `f`) searches the goal for an unique eligible application of `f`, and uses
+(with no arguments to `f`) searches the goal for a unique eligible application of `f`, and uses
 these arguments. An application of `f` is eligible if it is saturated and the arguments that will
 become targets are free variables.
 
@@ -1058,7 +1058,7 @@ The form
 ```
 fun_cases f
 ```
-(with no arguments to `f`) searches the goal for an unique eligible application of `f`, and uses
+(with no arguments to `f`) searches the goal for a unique eligible application of `f`, and uses
 these arguments. An application of `f` is eligible if it is saturated and the arguments that will
 become targets are free variables.