PUBDEV-8279: Fixed typo in replacing-values.rst code example docs

[rwbrown002]

https://h2oai.atlassian.net/browse/PUBDEV-8279

Fix typos in code example at h2o-docs/src/product/data-munging/replacing-values.rst

1. Previous example showed the following typo, using an assignment operator (<-) rather than less than symbol (<) to replace values less than 4.4

   df[df[, "sepal_len"] <- 4.4, "sepal_len"] <- 4.6

2. Previous example demonstrates manipulating entire column with df[,1] but says "second column" in the description. Correct to say "first" column. Indexes start at 1.

[michalkurka]

thank you for your contribution @rwbrown002

[michalkurka]

@hannah-tillman the changes look good - however - can you please check the consistency of the example as a whole?