Fixed error in badge.

Author: dprohe

README.rst

@@ -11,7 +11,7 @@ It also contains various structural dynamics and signal processing tools.
 ..
     Badges ========================================================================
 
-.. |documentation|:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/pages/main?label=Documentation
+.. |documentation| image:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/pages/main?label=Documentation
     :target: https://sandialabs.github.io/sdynpy/
 
 .. |build| image:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/main?label=GitHub&logo=github

docs/source/index.rst

@@ -47,7 +47,7 @@ Indices and tables
 ..
     Badges ========================================================================
 
-.. |documentation|:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/pages/main?label=Documentation
+.. |documentation| image:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/pages/main?label=Documentation
     :target: https://sandialabs.github.io/sdynpy/
 
 .. |build| image:: https://img.shields.io/github/workflow/status/sandialabs/sdynpy/main?label=GitHub&logo=github