Fixed clunky grammar, that was difficult to read.

src/ch08-02-strings.md

@@ -317,10 +317,10 @@ index to determine how many valid characters there were.
 
 ### Slicing Strings
 
-Indexing into a string is often a bad idea because it’s not clear what the
-return type of the string-indexing operation should be: a byte value, a
-character, a grapheme cluster, or a string slice. If you really need to use
-indices to create string slices, therefore, Rust asks you to be more specific.
+Indexing into a string can be problematic because it's unclear what the return 
+type of the string-indexing operation should be: a byte value, a character, 
+a grapheme cluster, or a string slice. If you need to use indices to create 
+string slices, Rust requires you to be more specific.
 
 Rather than indexing using `[]` with a single number, you can use `[]` with a
 range to create a string slice containing particular bytes: