Fix documentation issues

the following warnings appear whn docs are built:

asciidoctor: WARNING: parts/execspec.adoc: line 72: tag 'executable' not found in include file: /Users/schalkc/Projects/VariousGradlePlugins/grolifant/src/gradleTest/runtimeCompatibility/src/test/groovy/org/ysb33r/grolifant/compatibility/testing/AbstractCommandExecSpecSpec.groovy
asciidoctor: WARNING: parts/execspec.adoc: line 73: no callouts refer to list item 1
asciidoctor: WARNING: parts/execspec.adoc: line 74: no callouts refer to list item 2
asciidoctor: WARNING: parts/execspec.adoc: line 76: no callouts refer to list item 3