Update src/items/functions.md

Co-Authored-By: Mazdak Farrokhzad <[email protected]>

Author: nikomatsakis

src/items/functions.md

@@ -287,7 +287,7 @@ other unsafe function, these conditions may extend beyond the initial
 call itself -- in the snippet above, for example, the `unsafe_example`
 function took a pointer `x` as argument, and then (when awaited)
 dereferenced that pointer. This implies that `x` would have to be
-valid until the future is finished execution, and it is the callers
+valid until the future is finished executing, and it is the callers
 job to ensure that.
 
 ## Attributes on functions