Fix Vale issues integration in doc/integration/mattermost/index.md

What does this MR do?

This MR is going to resolve this issue #346862 (closed). In this one, Vale tool found these cases :

21:85 Avoid using future tense "will need". Use present tense instead.
44:5 Avoid using future tense "will need". Use present tense instead.
54:153 Avoid using future tense "will be". Use present tense instead.
74:42 Avoid using future tense "you'll need". Use present tense instead.
88:93 Avoid using future tense "will still". Use present tense instead.
88:157 Avoid using future tense "will not". Use present tense instead.
127:37 Avoid using future tense "will be". Use present tense instead.
193:63 Avoid using future tense "will need". Use present tense instead.
232:10 Avoid using future tense "will be". Use present tense instead.
302:71 Avoid using future tense "going to the". Use present tense instead.

but i keep these :

88:157 Avoid using future tense "will not". Use present tense instead
232:10 Avoid using future tense "will be". Use present tense instead.
302:71 Avoid using future tense "going to the". Use present tense instead.

Are ok for this ?

Related issues

#346862 (closed)

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