fixed linting

Author: hechth

tools/histogram/.shed.yml

@@ -7,4 +7,5 @@ long_description: |
 name: histogram
 owner: devteam
 remote_repository_url: https://github.com/galaxyproject/tools-devteam/tree/master/tools/histogram
+homepage_url: https://github.com/galaxyproject/tools-devteam/tree/master/tools/histogram
 type: unrestricted

tools/histogram/histogram2.xml

@@ -84,4 +84,7 @@ This tool computes a histogram of the numerical values in a column of a dataset.
 .. image:: histogram2.png
 
 </help>
+<citations>
+  <citation type="doi">10.1145/3386334</citation>
+</citations>
 </tool>