Correct inconsistencies in README and CONTRIBUTING docs (rapidsai#682)

This PR corrects a couple of inconsistencies in the documentation.

Authors:
  - Robert Maynard (@robertmaynard)

Approvers:
  - Keith Kraus (@kkraus14)

URL: rapidsai#682

Author: robertmaynard

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing to RMM
 
-If you are interested in contributing to cuDF, your contributions will fall
+If you are interested in contributing to RMM, your contributions will fall
 into three categories:
 1. You want to report a bug, feature request, or documentation issue
     - File an [issue](https://github.com/rapidsai/rmm/issues/new/choose)

README.md

@@ -56,7 +56,7 @@ Compiler requirements:
 
 * `gcc`     version 7.0 or higher required
 * `nvcc`    version 9.0 or higher recommended
-* `cmake`   version 3.12 or higher
+* `cmake`   version 3.18 or higher
 
 CUDA/GPU requirements:
 
@@ -88,7 +88,7 @@ $ source activate cudf_dev
 ```
 
 - Build and install `librmm` using cmake & make. CMake depends on the `nvcc` executable being on
-  your path or defined in `$CUDACXX`.
+  your path or defined in `CUDACXX` environment variable.
 
 ```bash
 
@@ -101,7 +101,7 @@ $ make install                                      # install the library librmm
 
 - Building and installing `librmm` and `rmm` using build.sh. Build.sh creates build dir at root of
   git repository. build.sh depends on the `nvcc` executable being on your path or defined in
-  `$CUDACXX`.
+  `CUDACXX` environment variable.
 
 ```bash