Remove redundant regex testers

[slevithan]

• Removes regextester.com and regexpal.com
  • Both domain names are aliases for the same webapp.
  • It is (they are) a pretty good tool, but not nearly as good as the other two regex testers already listed above them (regex101 and RegExr). It (they) adds no additional features, apart from extra ads. So they are redundant in this list.
• Fixes the capitalization of regex101's name.
• Tweaks the descriptions of regex101 and RegExr (e.g. to add additional regex flavors supported by regex101).