Fix Typos: welcome_and_next_steps.txt corrections

[Sandplum]

"a great time to to join"

remove to where it is repeated

"poping by weekly open house"

poping s/b popping, weekly s/b the weekly

"recommendable" s/b "recommended"

"Be happy to ask if you like to know more!"

s/b Feel free to ask if you would like to know more!

These phrases are viewed in the email sent to new members.
The phrases come from welcome_and_next_steps.txt https://github.com/TampereHacklab/mulysa/blob/ceb692dd94d2bfe7ae6192c3a41a35dfbd0a8b37/users/templates/mail/welcome_and_next_steps.txt#L8C21-L8C44

Note: this email is also translated into Finnish.

[Sandplum]

See also: users/templates/mail/application_received.txt