spec: Expand "OCI" in spec-title reference

For consistency with the rest of the spec:

  $ git --no-pager grep --count 'Open Container Initiative Runtime Specification'
  README.md:1
  config.md:2
  schema/config-schema.json:1
  schema/defs.json:2
  spec.md:1
  specs-go/config.go:1
  $ git --no-pager grep --count 'OCI Runtime Specification'
  spec.md:1

Signed-off-by: W. Trevor King <wking@tremily.us>

spec.md

@@ -4,7 +4,7 @@ The [Open Container Initiative][oci] develops specifications for standards on Op
 
 # <a name="ociRuntimeSpecAbstract" />Abstract
 
-The OCI Runtime Specification aims to specify the configuration, execution environment, and lifecycle of a container.
+The Open Container Initiative Runtime Specification aims to specify the configuration, execution environment, and lifecycle of a container.
 
 A container's configuration is specified as the `config.json` for the supported platforms and details the fields that enable the creation of a container.
 The execution environment is specified to ensure that applications running inside a container have a consistent environment between runtimes along with common actions defined for the container's lifecycle.