Fix typo

docs/singularity.md

@@ -446,6 +446,6 @@ the container instead.
 There is, however, another solution: using Singularity inside Docker. By
 creating a bare-bones, Linux-based Docker image with Singularity you can build
 Singularity images locally on non-Linux operating systems. This can be either
-from a Singularity definition files or directly from already existing Docker
+from Singularity definition files or directly from already existing Docker
 images. You can read more about this at the following
 [GitHub repository](https://github.com/fasterius/singularity-in-docker).