Fix a typo. (#452)

* Fix a typo.

* Pin black version in travis

Co-authored-by: Adam Urbańczyk <adam-urbanczyk@users.noreply.github.com>

.travis.yml

@@ -41,6 +41,7 @@ matrix:
     env: PYTHON_VERSION=3.7
     os: linux
     script:
+    - pip install black==19.10b0
     - black . --diff --check
     - mypy cadquery
 

doc/primer.rst

@@ -129,7 +129,7 @@ the core CQ methods to get at the objects that were created.
 Each time a new CadQuery object is produced during these chained calls, it has a ``parent`` attribute that points
 to the CadQuery object that created it. Several CadQuery methods search this parent chain, for example when searching
 for the context solid. You can also give a CadQuery object a tag, and further down your chain of CadQuery calls you
-can refer back to this particular object using it's tag.
+can refer back to this particular object using its tag.
 
 
 The Context Solid