Fix typos (#1079)

Author: spastorino

src/borrow_check/region_inference/placeholders_and_universes.md

@@ -21,7 +21,7 @@ fn main() {
 ```
 
 This program ought not to type-check: `foo` needs a static reference
-for its argument, and `bar` wants to be given a function that that
+for its argument, and `bar` wants to be given a function that
 accepts **any** reference (so it can call it with something on its
 stack, for example). But *how* do we reject it and *why*?
 

src/contributing.md

@@ -191,7 +191,7 @@ git subtree pull -P src/tools/clippy https://github.com/rust-lang/rust-clippy ma
 ```
 
 takes all changes since the last `subtree pull` from the tool repo
-repo and adds these commits to the rustc repo + a merge commit that moves the tool changes into
+and adds these commits to the rustc repo + a merge commit that moves the tool changes into
 the specified directory in the rust repository.
 
 It is recommended that you always do a push first and get that merged to the tool master branch.