[Petal] Minor documentation bug in Petal.pm

[Bruno Postle]

The pod documentation refers to Petal::Disk::Cache and
Petal::Disk::Memory modules (which don't exist).

-- 
Bruno

134c134
< $DISK_CACHE  - If set to FALSE, Petal will not use the Petal::Disk::Cache module. Defaults to TRUE.
---
> $DISK_CACHE  - If set to FALSE, Petal will not use the Petal::Cache::Disk module. Defaults to TRUE.
142c142
< $MEMORY_CACHE - If set to FALSE, Petal will not use the Petal::Disk::Memory module. Defaults to TRUE.
---
> $MEMORY_CACHE - If set to FALSE, Petal will not use the Petal::Cache::Memory module. Defaults to TRUE.