Fix repository_tool documentation header

[trishankkarthik]

The header "Create TUF Repository" is a bit misleading, because the code example is not creating a new repository, but rather loading an old one.

We should either change the heading to reflect the example, or change the example to reflect the header.

[vladimir-v-diaz]

You should report issues in the pull request.
#170

That header contains many sub-sections, not just that introductory example. The header formatting might not be clear.

Create TUF Repository
    Keys
    Create a new Repository
    Delegations
    etc.
Client Setup

[vladimir-v-diaz]

Table of Contents added: 511382b