Fix typos in Contributors Guide
@@ -249,7 +249,7 @@ There are four main components to PyGMT's documentation:
* Getting started/developer documentation, with source text in ReST `*.rst`
and markdown `*.md` files under the `doc/` folder.
-The documentation are written primarily in
+The documentation is written primarily in
[reStructuredText](https://docutils.sourceforge.io/rst.html) and built by
[Sphinx](http://www.sphinx-doc.org/). Please refer to
{gmt-docs}`reStructuredText Cheatsheet <devdocs/rst-cheatsheet.html>`
@@ -363,7 +363,7 @@ General guidelines for making a good gallery plot:
* Use a sample dataset from `pygmt.datasets` if you need to plot data. If a suitable
dataset isn't available, open an issue requesting one and we'll work together to add
it.
-* Add comments to explain things are aren't obvious from reading the code. Good: *Use a
+* Add comments to explain things that aren't obvious from reading the code. Good: *Use a
Mercator projection and make the plot 15 centimeters wide*. Bad: *Draw coastlines and
plot the data*.
* Describe the feature that you're showcasing and link to other relevant parts of the
