Fixed error building docs

.travis.yml

@@ -216,7 +216,7 @@ script:
   - python devtools/run-ctest.py --start-time $START_TIME
 
   - if [[ ! -z "${DOCS_DEPLOY}" && "${DOCS_DEPLOY}" = "true" ]]; then
-      pip install sphinx sphinxcontrib-bibtex sphinxcontrib-lunrsearch sphinxcontrib-autodoc_doxygen;
+      pip install sphinx==2.3.1 sphinxcontrib-bibtex sphinxcontrib-lunrsearch sphinxcontrib-autodoc_doxygen;
       make sphinxhtml;
       make sphinxpdf;
       make C++ApiDocs PythonApiDocs;

openmmapi/include/openmm/Integrator.h

@@ -87,13 +87,13 @@ public:
     virtual void step(int steps) = 0;
     /**
      * Get which force groups to use for integration.  By default, all force groups
-     * are included.  This is interpreted as a set if bit flags: the forces from group i
+     * are included.  This is interpreted as a set of bit flags: the forces from group i
      * will be included if (groups&(1<<i)) != 0.
      */
     virtual int getIntegrationForceGroups() const;
     /**
      * Set which force groups to use for integration.  By default, all force groups
-     * are included.  This is interpreted as a set if bit flags: the forces from group i
+     * are included.  This is interpreted as a set of bit flags: the forces from group i
      * will be included if (groups&(1<<i)) != 0.
      */
     virtual void setIntegrationForceGroups(int groups);