[Testing] fix 'eachother' -> 'each other'

[nealio82]

'each other' is two separate words, see https://dictionary.cambridge.org/grammar/british-grammar/each-other-one-another

[javiereguiluz]

Thanks Neal!

Just a minor note: in Symfony Docs we use "American English" and, according to our documentation standards, the reference dictionary is the American English Oxford Dictionary. In any case, the fix is correct also for "American English", so we're merging this and we thank you for fixing this!