Fixed missing `

CppCoreGuidelines.md

@@ -1010,7 +1010,7 @@ Instead, we could use `vector`:
 
 The standards library and the GSL are examples of this philosophy.
 For example, instead of messing with the arrays, unions, cast, tricky lifetime issues, `gsl::owner`, etc.
-that is needed to implement key abstractions, such as `vector`, `span`, `lock_guard, and `future`, we use the libraries
+that is needed to implement key abstractions, such as `vector`, `span`, `lock_guard`, and `future`, we use the libraries
 designed and implemented by people with more time and expertise than we usually have.
 Similarly, we can and should design and implement more specialized libraries, rather than leaving the users (often ourselves)
 with the challenge of repeatedly getting low-level code well.