Fix typos in RFCs 751-1000

Author: jhpratt

text/0769-sound-generic-drop.md

@@ -4,7 +4,7 @@
 
 # History
 
-2015.09.18 -- This RFC was partially superceded by [RFC 1238], which
+2015.09.18 -- This RFC was partially superseded by [RFC 1238], which
 removed the parametricity-based reasoning in favor of an attribute.
 
 [RFC 1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md

text/0953-op-assign.md

@@ -42,7 +42,7 @@ Implement these traits for the primitive numeric types *without* overloading,
 i.e. only `impl AddAssign<i32> for i32 { .. }`.
 
 Add an `op_assign` feature gate. When the feature gate is enabled, the compiler
-will consider these traits when typecheking `a += b`. Without the feature gate
+will consider these traits when typechecking `a += b`. Without the feature gate
 the compiler will enforce that `a` and `b` must be primitives of the same
 type/category as it does today.
 

text/0980-read-exact.md

@@ -56,7 +56,7 @@ The following method is added to the `Read` trait:
 fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>;
 ```
 
-Aditionally, a default implementation of this method is provided:
+Additionally, a default implementation of this method is provided:
 
 ``` rust
 fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {