Fix spelling mistakes in documentation

While reading  some of the docs I noticed a few errors, so I ran
misspellings (https://pypi.python.org/pypi/misspellings) on markdown files

Signed-off-by: Joe Gordon <joe.gordon0@gmail.com>

image/spec/v1.md

@@ -176,7 +176,7 @@ Here is an example image JSON file:
         should be omitted. A collection of images may share many of the same
         ancestor layers. This organizational structure is strictly a tree with
         any one layer having either no parent or a single parent and zero or
-        more decendent layers. Cycles are not allowed and implementations
+        more descendent layers. Cycles are not allowed and implementations
         should be careful to avoid creating them or iterating through a cycle
         indefinitely.
     </dd>