[docs] fix grammatical error in section title
@@ -56,7 +56,7 @@ file.
We support R versions 3.4, 3.5 and 3.6.
-Why is my repository is failing to build with ``ResolvePackageNotFound`` ?
+Why is my repository failing to build with ``ResolvePackageNotFound`` ?
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If you used ``conda env export`` to generate your ``environment.yml`` it will
